Manufacturing of electronic assemblies such as printed circuit boards requires testing after the components have been placed on the printed circuit board in order to determine the proper continuity of interconnections, placement and connection of components, and board function. Several different approaches have been developed for testing the components and printed circuit (P.C.) boards, including functional testing, in-circuit testing, and continuity testing.
Functional testing uses a procedure of applying predetermined input signals to a printed circuit board and monitoring the output of a printed circuit board via an edge connector to determine if all of the components are present and operating properly on the board. While functional testing provides a way of determining whether a printed circuit board is functioning properly, it provides little or no information regarding the functioning of individual components on the board. Complex programming techniques have been used to provide limited information as to the location of non-functioning components on the board by carefully selecting input data and analyzing the output results. Such systems are complex, often costly to implement, and normally provide only vague information as to the location of malfunctioning components. Therefore, if an edge-connector test indicates a faulty printed circuit board, it is frequently desirable to "backtrace" through the circuit from the failing output to find the source of the problem. Backtracing is frequently performed manually, which requires an operator to hold a probe in physical contact with a pad or other test point on the board. Today's fine pitch geometries make this extremely impractical.
Due to the limitations of functional testing, in-circuit testing techniques have been used to individually test each of the components on a printed circuit board to determine if these components are working properly. This testing method uses a "bed-of-nails" fixture employing fixed position spring-loaded probes which establish an ohmic contact by pressing against various test points on a printed circuit board or against the leads of each component in order to access and test each individual component. In this manner, non-functioning components can be readily identified and replaced, thus preventing the entire circuit board from being scrapped. This process works well for simple components where the circuit inside the component is known and can be easily tested. Also, because the devices are independently tested, tests for many common digital integrated circuits can be programmed once, in advance, stored in a library and then called upon when needed. This greatly simplifies test generation, since this preprogrammed test can be used over and over again. However, if the component being tested is very complex, or if the circuit inside the component is unknown, in-circuit testing may not achieve satisfactory results.
Also, integrated circuit packages have evolved from packages with 16-20 leads spaced 0.1" (2.5 mm) apart to packages which have hundreds of leads with spacing of 0.025" (0.6 mm). These state-of-the-art technologies have increased packaging densities at the cost of test accessibility. As the number of leads on integrated circuit packages continues to increase and the spacing between leads (pitch) continues to decrease, the design and manufacture of test probes and fixtures becomes more and more difficult. This is exasperated by the fact that manufacturing variances can be as much as or even exceed current lead pitches, making it difficult for a bed-of-nails test fixture to accurately access the leads and components of every printed circuit board. This is complicated further by assemblies which have components placed on both sides so that double-sided "clam-shell" fixtures are required, which still may not be able to access all of the components that are required to be tested. Accordingly, there is a need for a different approach to testing fine-pitch components. In particular, instead of probes with fixed positions, probes need to be moveable to accommodate various manufacturing tolerances in assemblies and fixtures. Alternatively, some assembly designs need a hybrid approach using a combination of fixed and moveable probes.
Another disadvantage of bed-of-nails test fixtures is that designing one (or two in the case of double sided P.C. boards) bed-of-nails test fixtures for each printed circuit board can be extremely expensive. As the test fixtures grow in complexity, it becomes more and more difficult to determine if the test fixtures were properly built or if there are any incorrectly wired test probes. This is not just a problem of testing for initial fixture manufacturing defects, but there is an on going need for diagnosis of faults in fixtures which may appear later. Accordingly, there is a need for flexible, programmable testing of test fixtures.
One recent solution to the complexity and expense of dedicated bed-of-nails testers is a robotic tester. For example, in U.S. Pat. No. 5,107,206, by Yanagi et al., four spring probes are mechanically positioned over a printed circuit board so that components can be tested. Prior art FIGS. 1 and 2 illustrate the manner in which test probes 11-15 electrically contact leads 16-20 of integrated circuit 21 and solder joints 22-25, which connect the integrated circuit 21 to printed circuit board 26.
The disadvantages of this approach are two fold. First of all, the test is much slower than the standard bed-of-nails tester, due to the added time for positioning the probes and the fact that for most individual tests all of the probes must be used--one for excitation, one for measuring, and one or more for grounding upstream elements that could be electrically damaged by the test. A second disadvantage is that the robot is programmed to bring the test probes down at predefined test points. These predefined test points may actually be slightly different from the intended test points due to manufacturing variances, causing the robot to miss its intended test point. When the robot "misses" it can cause damage to the printed circuit board, the wiring traces, the component leads, and the components themselves. Because the spring probes must come into intimate electrical contact with the elements on the printed circuit board, damage can occur even with proper alignment of the board under test. As can be appreciated from viewing FIGS. 1 and 2, there is very little room for manufacturing variances with the test system of Yanagi et al. Accordingly, there is a need for a test system that can perform tests without the expense of elaborate, individually dedicated bed-of-nails fixtures and without damaging the printed circuit board or the components thereon.
Another very important potential problem that must be tested for on every printed circuit board is whether all the pins of every component are in fact soldered to the circuit board. Functional testing may miss a particular pin, if the functions performed by that particular pin are not thoroughly tested in the functional test. Testing for this type of fault is particularly difficult when the circuit inside the component is unknown, such as the case with application specific integrated circuits (ASICs). Because of the large number of ASICs and the complexity of these devices, it is often not feasible to design a functional test to isolate each component or lead. And, in-circuit testing may give false responses during this type of test, because the spring probes can push the leads of the components down onto the printed circuit board, creating a temporary electrical connection with the wire trace on the printed circuit board, thus falsely indicating a good solder joint.
One recent approach to continuity testing is to stimulate the circuit and then use a non-contact test probe elsewhere in the circuit to determine continuity, or vice versa. Some of these methods include measuring of electromagnetic energy, inductance, capacitance, and thermal energy. For example, in U.S. Pat. No. 5,111,137, electromagnetic radiation is used to induce changes in the current or voltage of a device under test. In U.S. Pat. No. 5,124,660, a capacitive probe is used to induce changes in the current or voltage of a device under test. These probes are especially useful for testing the continuity of individual connections such as wire bonds or soldered connections. These probes may be built into a bed-of-nails test fixture with a single probe for each individual connection to be tested. This approach overcomes the damage that spring probes used in bed-of-nails fixtures and robots can cause to the components and printed circuit boards, but dedicated fixtures utilizing non-contact test probes are just as complex and expensive as ohmic bed-of-nails fixtures. Accordingly, for fine pitch components, an alternative, less expensive approach to the non-contact test probe fixture is needed in which the secondary stimulus and associated test probes are moved precisely to the connection to be tested.
Finally, sometimes a manufacturer or a value added supplier may wish to perform one or more of the post-assembly tests discussed above. This can be very expensive as most of these tests are performed by different machines and many of these tests require individually dedicated test fixtures, libraries, and programs. Accordingly, there is a need for a test system that can perform multiple post-assembly tests with the least amount of dedicated or redundant equipment, libraries and programs as possible.